Maze Blaze

This is the description once unlocked.
In co-op mode get trapped behind the wall erected by the Admiral

The admiral is the final boss, you face him on in the 4th room. I don't know what triggers this but he does an attack where he puts up a circular crystal wall around him that blocks off the outer walled area of the arena. My second controller was outside the wall and I still unlocked the achievement. I was unable to break the wall or get back in either, however when playing solo and getting locked inside, it would go down after a bit. So you could potentially try this multiple times if you dont get it first try.

The wall triggered for me when he was at about 20% health. I had tried to take him slowly to bait the attack but was just met with minion spawns, but after I become more aggressive, the wall came up quick.

I did this solo with a second controller and I do not recommend. It took me about an hour running through the game as you have to constantly move both players and I pretty much only survived my run because I got the upgrade that gives you med kits when getting multikills. Also during the boss fight, it seems if someone stays behind the outer wall for cover, constant minion spawns happen. I barely survived this and I feel I got lucky.